A better understanding of the effects of alloying Pt with Au on the oxygen reduction reaction (ORR) was achieved in this study. Recent investigations of carbon-supported Au-Pt nanoparticle catalysts have shown enhanced response for the ORR with respect to pure Pt nanoparticles. These results were reported to be due to homogeneous mixing of Au and Pt atoms that was accomplished in nanoscale particles, effectively removing the miscibility gap inherent in the equilibrium phase diagram. In this study, cosputtering of Au and Pt is used to form 350 nm thick films to achieve homogeneous mixing without nanoparticle geometries, thereby isolating the effects of alloying from nanoparticle effects. ORR activity and stability at these electrodes are investigated via cyclic voltammetry in O-2-saturated 0.5 M H2SO4 under oxidizing potentials. Cosputtering yielded solid solution alloys throughout the entire composition range as shown by X-ray diffraction results. Furthermore, as-deposited near-surface compositions determined by X-ray photoelectron spectroscopy indicated that extensive Au surface segregation was avoided by room-temperature deposition. Metastable compositions within the miscibility gap, explicitly Pt21Au79, Pt43Au57, and Pt59Au41, were found to be significantly more electrocatalytic for O-2 reduction than the pure Pt film, although Pt43Au57 and Pt59Au41 rapidly lost catalytic activity with subsequent potential sweeps. (C) 2008 The Electrochemical Society.